Dielectric materials

ABSTRACT

Dielectric materials having low dielectric constant are provided, the materials comprising fabrics impregnated with thermosetting resins, at least a portion of the fibers in the fabrics being fluorocarbon fibers. Also provided is a method of manufacture of these dielectric materials. The dielectric materials are useful in the fabrication of multilayer printed circuit boards.

This application is a divisional of copending U.S. patent applicationSer. No. 705,538, filed Mar. 26, 1985, now U.S. Pat. No. 4,680,220.

BACKGROUND OF THE INVENTION

The present invention is related to low dielectric constant dielectricmaterials suitable for use, without limitation, in multilayer printedcircuit boards.

The principal conventional dielectric material in present use forprinted circuit boards is a laminated composite comprised of fiberglassfabric impregnated with a thermosetting epoxy resin, referred to by theNational Electronic Manufacturers Association (NEMA) classification asFR-₄. FR-₄ is produced by impregnating fiberglass fabric with a liquid,thermosetting epoxy resin. The resin in the impregnated fabric ispartially cured with heat to form a dry, flexible sheet in which theresin is in an intermediate cure state, termed "B"-stage or "pre-preg"sheet. One or more sheets of pre-preg are then stacked together to adesired thickness and laminated together by further curing under heatand pressure to form a laminated composite in which the resin is in afully-cured state, termed the "C"-stage state.

During the lamination process, the B-stage epoxy resin of the pre-pregsheet is converted to fully-cured C-stage resin. Normally, the sheets ofpre-preg are bonded to one or two sheets of copper foil during thelamination process so that the laminated composite consists ofdielectric material clad on one or both sides with copper foil. Thiscomposite material is referred to as the FR-₄ copper clad laminate andis fabricated into single and double sided printed circuit boards.

Where very high circuit densities are required, printed circuit boardswith more than two layers of circuitry have been developed, calledmultilayer printed circuit boards. Thin dielectric FR-₄ copper cladlaminate is fabricated into single or double sided circuit patternscalled innerlayers. One or more of these innerlayers are interleavedwith one or more sheets of B-stage pre preg and laminated together underheat and pressure to form a homogeneous, void free multilayer structure.The lamination process converts the B-stage epoxy resin of the pre-preginto C-stage resin, bonding the innerlayers together and providinginsulation between the circuit layers. The multilayered structure isfurther processed into a multilayer printed circuit board.

The thermosetting resin is essential to the fabrication of multilayerprinted circuit boards in which the resin is uniform throughout. TheB-stage resin in the pre-preg is converted to a fully-cured C-stagestate without melting or materially altering the C-stage dielectricmaterial in the innerlayers. Because the intermediately cured pre-pregis essentially the same composition as the C-stage resin in theinnerlayers, the multilayer composite is dimensionally stable and easilyprocessed.

There is a need in the electronics industry for dielectric materialshaving lower dielectric constants than that of convention materialsbecause signal speeds and operating frequencies of electric systems haveincreased dramatically. Lower dielectric constants dielectric materialsboth decrease capacitive coupling and increase the speed of theelectronic signal so that electronic systems can process data at greaterspeeds.

FR-₄ laminate has a relatively high dielectric constant, approximately5.0 at 1 megahertz. This is a result of the high dielectric constantcontribution of the fiberglass, 6.11, averaged with the lower dielectricconstant of the epoxy resin, 3.4. To achieve a lower dielectric constantdielectric material, the electronics industry has turned to laminatedcomposites comprised of fiberglass fabric impregnated with fluorocarbonresins. These laminated composites have a dielectric constant ofapproximately 2.5 at 1 megahertz. However, fluorocabrons are notthermosetting resins and are extremely difficult to fabricate intomultilayer printed circuit boards. At temperatures at which thefluorocarbon pre-preg sheets will bond the package together, theinnerlayers can melt or lose their dimensional stability. If FR-₄pre-pregs are used to bond the fluorocarbon innerlayers together, theresulting multilayer composite is non-homogeneous and the dielectricconstant is raised as a result of the higher dielectric constant of thepre-preg.

Other specialty dielectric materials have been developed which utilizefibers other than fiberglass in combination with thermosetting resins.Laminated composites of polyaramide fibers and epoxy resins have adielectric constant of 3.8, which is considerably higher than thefluorocarbon composites. Quartz fibers have been used in composites, butquartz fibers have nearly the same dielectric constant as polyaramidefibers.

SUMMARY OF THE INVENTION

A dielectric material is provided comprising a fabric having fibers andinterstices between the fibers, in which at least a portion of thefibers in the fabric are fluorocarbon fibers, the fabric beingimpregnated within the interstices between the fibers with athermosetting resin cured to at least the semi-cured, B-stage state, thedielectric constant of this dielectric material being less than 3.5,preferably less than 3.0. The thermosetting resin may be cured to thefully-cured, C-stage state and the dielectric material may haveelectrically conductive foil bonded to at least one surface thereof.

Also provided is a laminated composite structure comprising one or moresheets of this dielectric material in which the thermosetting resin iscured to the fully-cured, C-stage state, and one or more sheets of thedielectric material in which the thermosetting resin is cured to thesemi-cured, B-stage state, oriented such that, wherever a layer ofmaterial in the C-stage state contacts another layer, the other layer isof material in the B-stage state.

In a laminated composite structure comprising one or more sheets of thisdielectric material in which the thermosetting resin is cured to thefully-cured, C-stage state, and having an electrically conductive foilbonded to at least one side of at least one of the sheets, and one ormore sheets of the dielectric material in which the thermosetting resinis cured to the semi-cured, B-stage state, the layers are oriented suchthat, wherever a layer of material in the C-stage state, and wherever anelectrically conductive foil contacts another layer, the other layer isof material in the B-stage state.

In use for electrical applications, the thermosetting resin in thedielectric material is cured to the fully-cured, C-stage state,throughout the laminated composite. The composite structure may have atleast one additional layer of a material other than the dielectricmaterial of this invention. This additional layer may be a layer ofepoxy/fiberglass, fluorocarbon/fiberglass, polyimide/fiberglass,epoxy/polyaramide fiber, epoxy/quartz fiber and polyimide/quartz fibercomposites. The thermosetting resin may be an epoxy, polymide,polyamide, polyester, phenolic or an acrylic thermosetting resin. It ispreferably an epoxy resin. All fibers in the fabric can be fluorocarbonfibers or combinations of fluorocarbon fibers with fiberglass fibers,polyaramide fibers, or quartz fibers. The fabric may be a woven fabricor a non-woven felt or mat. The fluorocarbon fibers are PTFE fiberswhich may be drawn, nonporous, sintered PTFE fibers or expanded, porousPTFE fibers. The fluorocarbon fibers may contain a filler which can be adielectric insulator material such as ceramic or glass, or it may beelectrically conductive such as electrically conductive orsemi-conductive metals, metal oxides or carbon. The dielectric materialis useful in a printed circuit laminated composite, as a dielectricinsulator for submicrowave or microwave signals. It may be used in amicrowave printed circuit board and in a microwave radome.

Also provided is a process for making the dielectric materialcomprising: (a) treating fluorocarbon fibers to render them wettable byan uncured thermosetting resin, the fibers being held under longitudinaltension during such treatment, thereby preventing longitudinal shrinkageof the fibers during treatment, (b) weaving the fabric containing atleast a portion of the treated fluorocarbon fibers, (c) impregnating thefabric with an uncured thermosetting resin, and (d) heating and dryingthe resin-impregnated-fabric to cure the resin to at least thesemi-cured, B-stage state. Alternatively, the process for making thedielectric material may comprise the steps of: (a) treating a fabriccontaining at least a portion of fluorocarbon fibers to render thefluorocarbon fibers wettable by an uncured thermosetting resin, thefabric being restrained from shrinkage during such treatment or,alternatively, the fabric being allowed to shrink during treatment andstretched to nearly its original dimensions after treatment, (b)impregnating the fabric with an uncured thermosetting resin, and (c)heating and drying the resin-impregnated-fabric to cure the resin to atleast the semi-cured, B-stage state. The process may include the step ofcalendering the fabric to reduce its thickness. The treatment to makethe fluorocarbon fibers wettable includes applying an alkali naphthanatesolution to the fluorocarbon fibers or fabric. The process may includenapping the fluorocarbon fibers to improve wettability and adhesionbetween fibers and resin. The fibers or fabric can be bleachedsubsequent to the treatment to make them wettable, in order to lightenthe color of the fibers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic conceptual view of a process for making thedielectric material of this invention.

FIG. 1B is a schematic diagram of a process for treating a fluorocarbonfiber which can be woven into a fabric, shown in FIG. 1C, and which canbe processed in a similar manner as in FIG. 1 to make the dielectricmaterial of this invention.

FIG. 2 shows a stack of pre-preg sheets of this invention having sheetsof metal foil above and below the pre-preg sheets.

FIG. 3 shows the result of applying heat and pressure to the sheets ofFIG. 2, to produce a composite having fully-cured, C-stage resin and thecopper foil bonded thereto on both top and bottom.

FIG. 4 shows a fabric for use in the invention having all fluorocarbonfibers.

FIG. 5 shows a fabric for use in the invention having fluorocarbonfibers and fiberglass fibers.

FIG. 6 shows the dielectric material of the invention having a printedcircuit bonded thereto.

FIG. 7 shows a stack of metal foils, dielectric sheets and a centralprinted circuit board just prior to lamination and bonding.

FIG. 8 is a diagram of the stack shown in FIG. 7 after being bonded byapplication of heat and pressure.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS WITHREFERENCE TO THE DRAWINGS

Dielectric materials having low dielectric constant are provided, thematerials comprising fabrics impregnated with thermosetting resins, atleast a portion of the fibers in the fabrics being fluorocarbon fibers.Also provided is a method of manufacture of these dielectric materials.The dielectric materials are useful in the fabrication of multilayerprinted circuit boards.

An objective of this invention is to provide dielectric materials havinga dielectric constant below 3.5 at 1 megahertz while exhibiting thedesirable handling and fabricating characteristics of structuresfabricated from thermosetting resins. A second objective is to provide asystem of pre-preg sheets and innerlayer printed circuit laminateshaving a dielectric constant below 3.5 at 1 megahertz which can befabricated into a homogeneous, void free, multilayer printed circuit. Afurther objective is to provide an economical process for fabricatingthese dielectric materials.

These objectives are accomplished by providing dielectric materialscomprised of thermosetting resins reinforced with fluorocarbon fibers.More specifically, fluorocarbon fibers are treated under longitudinaltension, preferably with an alkali napthanate solution, to provide awettable surface. The longitudinal tensioning of the fiber duringtreatment with an alkali napthanate is essential because, surprisingly,the fluorocarbon fiber shrinks up to approximately 20% during thetreatment process if left unrestrained. The treated fiber is woven intoa fabric using conventional weaving practices.

Untreated fluorocarbon fabric can not be impregnated with liquidthermosetting resins by known techniques. However, treated fluorocarbonfabric can be impregnated with a liquid thermosetting resin andpartially cured to a dry, flexible sheet of B-stage pre-preg. One ormore of the B-stage pre-preg sheets can be stacked together to a desiredthickness and laminated under heat and pressure to form a laminatedcomposite of thermosetting resin and flurocarbon fabric. If epoxy isused as the thermosetting resin, then the fluorocarbon fabric laminatedcomposite has a dielectric constant at 35 volume % epoxy ofapproximately 2.6 at 1 megahertz. The sheet of fluorocarbon fiberpre-preg can be bonded to one or two sheets of copper foil during thelamination process so that the laminated composite consists ofdielectric material clad on one or both sides with copper foil. If epoxyis used as the thermosetting resin, then the resulting fluorocarbonfiber, copper clad laminate has a dielectric constant at 35 volume % ofepoxy of 2.6 at 1 megahertz. The fluorocarbon fiber, copper cladlaminate can be fabricated into a single or-double-sided printed circuitboard.

One or more innerlayers made from the fluorocarbon fiber, copper cladlaminate can be interleaved with one or more fluorocarbon fiber pre-pregsheets and laminated together under heat and pressure to form ahomogeneous, void free, multilayer structure. If epoxy is used as thethermosetting resin, then the multilayer structure has a nearlyhomogeneous resin composition when further processed into a multilayeredprinted circuit board.

This unique process of impregnating specially treated fluorocarbonfabrics with thermosetting resin has resulted in unique products whichexhibit a dielectric constant below 3.5 and retain the processing andhandling advantages of thermosetting resins. It is the only known methodof fabricating homogeneous, void free, multilayer printed circuit boardshaving a uniform dielectric constant, being always below 3.5 at 1megahertz.

Specific fluorocarbon fibers impart desirable physical properties to thelaminated composite. Fluorocarbon fibers of drawn, sinteredpolytetrafluoroethylene (PTFE) made according to the teachings of U.S.Pat. No. 2,772,444 can be utilized in the present invention. However,the preferred fluorocarbon fiber for this invention is expanded, porousPTFE fiber made according to the teachings of U.S. Pat. Nos. 3,953,566and 4,187,390. Laminated composites comprised of expanded PTFE fibershave higher flexural strength, higher tensile strength and a lowershrinkage after processing than laminates of drawn, sintered PTFE fiber.

It has also been found that very thin laminated composites can beprepared by calendering the fluorocarbon fabrics to a reduced thickness.Fabric woven from expanded PTFE fibers can be calendered to thicknessesof less than 0.0025 inches and two sheets of epoxy pre-preg from thisfabric can yield a thin laminate with a dielectric core of 0.005 inchthickness. Dielectric cores of this thickness are very desirable forinnerlayers and flexible circuits.

If it is desirable to increase the laminated composite's tensilestrength, flexural strength, shrinkage after processing, or thermalcoefficient of expansion, then other fibers than fluorocarbon fibers canbe included in the composite structure. For example, fiberglass andtreated fluorocarbon fibers can be woven into the same fabric and usedas reinforcement for the thermosetting resins. Laminated compositescomposed of epoxy resin and expanded PTFE fabric containing less than 10wt. % fiberglass have exhibited a dielectric constant of 2.8 at 1megahertz while significantly improving the laminate's flexuralstrength, tensile strength, shrinkage after processing and thermalcoefficient of expansion. A second method of including other fibers thanfluorocarbon fibers into the fluorocarbon composite is to interleavesheets of fiberglass pre-preg with sheets of treated fluorocarbon fiberpre-preg. While this method can also yield a low dielectric constantlaminated composite, it does not have the same desirable homogeneousstructure.

An unexpected additional benefit of the fluorocarbon fiber laminatedcomposites is their high flexibility. A laminated composite 0.008 inchesthick, for example, of two sheets of expanded PTFE fabric impregnatedwith epoxy resin, can be bent around a 0.1 inch radius mandrel withoutbreaking the PTFE fibers, while fiberglass fibers in an equivalentthickness FR₋ 4 laminate will break when the laminate is bent over anequivalent diameter mandrel. The fluorocarbon fiber laminated compositescan, therefore, be excellent dielectrics for flexible printed circuitboards.

When the fluorocarbon fibers are treated with an alkali napthanate, thesurface of the fibers turns dark brown. In applications where this coloris not desirable, the fibers can be bleached to a lighter color usingheat or an oxidizing agent such as chlorine bleach. The lighteningprocess must be controlled to insure that a wettable surface remains.

The detailed description of the invention is best provided by referenceto the accompanying drawings and the examples below. FIG. 1A shows aconceptual process for making the pre-preg sheets according to thisinvention. Fabric 10 containing at least a portion ofpolytetrafluoroethylene fibers is fed from a feed roll over guide roll12 into bath 14 containing the alkali napthanate treatment solution 16.Pinned guides 13 pierce the fabric and prevent its shrinkage in both thelongitudinal and transverse directions during passage through thetreatment solution 16. The treated fabric 10 emerges from the bath 16and passes over guide roll 12 in the direction indicated by the arrowand enters water wash 18 wherein excess treatment solution is washedaway. The fabric is then optionally bleached in chamber 19, using, forexample, a chlorine bleach solution. The treated fabric then passesbetween optional compression rolls 20 wherein it may be reduced inthickness if desired. Liquid resin 22 is applied to the fabric fromcontainer 22 which wets the fabric and penetrates into its interstices.The coated fabric is heated in heater 26 to cure the resin to at leastthe B-stage cured or pre-preg sheet 29, and this sheet is cut intoindividual pre-preg sheets 30 by cutter 28.

FIG. 1B shows an alternate process for treating fibers for use in afabric to make the pre preg sheets of the invention. A PTFE fiber 40 ispassed around first tension roll 42, over and around guide rolls 44which guide the fiber through the treatment solution 48 in bath 46. Thefiber emerges from the bath and passes around a second tension roll 50,rotating at substantially the same speed as the first tension roll 42,thereby preventing the fiber from shrinkage during treatment. Thetreated fiber is then formed into a fabric 52 as depicted in FIG. 1C, tobe used in making the pre-preg sheets according to the invention.

FIG. 2 shows a stack of the pre-preg sheets 30 of the invention with ametal foil 32, preferably copper, placed above and below the sheets 30.On application of heat and pressure as depicted in FIG. 3, the resin inpre-preg sheets 30 is cured to a homogeneous, C-stage cured state toform the composite 34 and the metal foils 32 are firmly bonded to thecomposite 34.

FIG. 4 shows a portion of a woven fabric in which all fibers 40 areporous PTFE fibers, and FIG. 5 shows a portion of a fabric in which aportion of the fibers 40 are porous PTFE and a portion are glass fibers38.

FIG. 6 depicts a composite 34 from which a portion of the metal frommetal foil 32 has been removed leaving electrical circuitry 60.

FIG. 7 shows the composite of FIG. 6 sandwiched between two pre-pregsheets 30 and having metal foils 32 above and below the pre-preg sheets30. Wherever a layer of material in which the resin is in the C-stagestate, i.e. layer 34, and wherever an electrically conductive foil orcircuit, i.e. 32 or 60, contacts another layer, i.e. layers 30, theother layer is of material in the B-stage state.

FIG. 8 shows a composite of this invention 80 obtained by applying heatand pressure to the stack shown in FIG. 7. The resin in the centerportion 36 has become homogeneous and fully cured to the C-stage state.The foils 32 are securely bonded to both sides of the composite 36.

The following examples are intended to be illustrative of the inventionbut will not limit the scope of the claims in any way.

EXAMPLE I

A fabric was woven using conventional procedures from fibers of expandedpolytetrafluoroethylene (PTFE) available commercial as GORE-TEX®expanded PTFE weaving fibers from W. L. Gore & Associates, Inc., Elkton,Md. The fabric construction had 52 (400 denier) fibers per inch in thelongitudinal direction and 52 (400 denier) fibers per inch in thetransverse direction. The fabric was cut into six, 6 inch×6 inch sheetsand immersed unrestrained for 30 seconds in an alkali napthanatesolution, available commercially under the trademark TETRA-ETCH®, alsoavailable from W.L. Gore & Associates, Inc., Elkton, Md. Following thistreatment, the fabric was washed in warm tap water and rinsed inacetone. The treatment with the TETRA-ETCH® solution caused the fibersto develop a dark brown color and resulted in the fabric shrinkingapproximately 20% in both the longitudinal and transverse directions.The fabric was stretched to nearly its original dimensions by graspingthe fabric on its edges and manually stretching the fabric.

A liquid epoxy resin was prepared using the guidelines of Dow ChemicalCompany product brochure #296-396-783 for Dow epoxy resin 521-A80. Whenliquid epoxy resin was coated on untreated PTFE fabric, the resin beadedup and would not wet or penetrate the interstices between the fibers.Conversely, when the liquid epoxy resin was coated on the treatedfabric, it wet the fabric and filled the interstices between the fibersto form a level, even coating over the fabric surface.

The six epoxy-coated treated fabric sheets were placed one at a time ina convection oven at 160° C., each for 4 minutes. When the sheets wereremoved from the oven and cooled, the epoxy resin was observed to havecompletely wetted the fabric and had been converted into a dry,flexible, semi-cured state known as B-stage pre-preg. The average resinpickup for each pre-preg sheet was 5 gms and the average thickness wasapproximately 0.14 inches.

The six pre-preg sheets were stacked one upon another and placed betweenFEP release sheets and stainless steel caul plates to form a laminationpackage. The package was placed in a Carver® platen press which had beenpreheated to 175° C. and a pressure of 100 psi was applied. After threeminutes the pressure was increased to 800 psi and the package was curedfor 30 minutes at 175° C. The heaters were turned off and the packagewas allowed to cool to room temperature while still under the 800 psipressure. The package was removed from the press and the laminatedcomposite was separated from the FEP release sheets and caul plates.

The laminated composite was fully cured to the C-stage state, wasapproximately 0.45 inches thick and exhibited excellent resin wettingthroughout the composite. There was no evidence of air entrapment,blistering, resin voids or delamination between the layers of fabric.Microscopic examination of cross sections of the laminated compositeshowed a uniform distribution of epoxy resin around the fibers, withinthe interstices of the fabric and between the layers of fabric.

EXAMPLE II

The same conditions and materials of Example I were used, except thatthe stack of pre-preg sheets was placed between sheets of 0.0014 inchthick copper foil instead of the FEP release sheets. After removing thecooled sample from the platen press, it was observed that the copperfoil was intimately and securely bonded to the composite core of C-stagecured epoxy resin and GORE-TEX® expanded PTFE fibers to form a structureknown as a double-sided, copper clad laminate. The total laminatethickness was approximately 0.045 inches and the core thickness wasapproximately 0.042 inches. Using a capacitance bridge, the dielectricconstant at 1 MHz of the laminate sample was determined to beapproximately 2.8. The calculated weight percent fiber in the core was63% and that of epoxy resin was 37%.

EXAMPLE III

A double-sided copper clad laminate was prepared as detailed in ExampleII and fabricated into a double-sided, plated-through-hole circuit boardusing standard techniques familiar to those skilled in the art. Thecircuit board was electrically conductive along circuit lines and acrossthe plated-through hole connections.

EXAMPLE IV

Eight pre-preg sheets were prepared as described in Example I with theexception that the fabric construction had 64 fibers per inch ofGORE-TEX® expanded PTFE fiber in the longitudinal direction and 60fibers per inch in the transverse direction. Three sheets ofcommercially available pre-preg FR-₄ made from B-stage epoxy resinreinforced with a 7628 fiberglass fabric were placed in the stack ofexpanded PTFE pre-preg sheets and copper foil in the following order:

A=0.0014 copper foil

B=epoxy/fiberglass pre-preg

C=epoxy/expanded PTFE fiber pre-preg

    ______________________________________                                        Number of Sheets Material                                                     ______________________________________                                        1                A                                                            1                B                                                            4                C                                                            1                B                                                            4                C                                                            1                B                                                            1                A                                                            ______________________________________                                    

This package was placed between stainless steel caul plates andprocessed into a double-sided copper clad laminate using the conditionsdescribed in Example II.

The C-stage cured laminate was void free and exhibited excellentadhesion between layers of fabric, copper foil and resin. The dielectriccore was 0.092 inches thick and was composed of 15 wt. % fiberglass, 54wt. % GORE-TEX® expanded PTFE fibers and 31% C-stage epoxy resin. Thedielectric constant of the composite was 2.9 at 1 MHz.

The addition of 15 wt. % fiberglass significantly improved themechanical properties of the composite as shown below:

Core Composition A=55 wt. % Epoxy/45 wt. % GORE-TEX® expanded PTFEfiber.

Core Composition B=15 wt. % Fiberglass/31 wt. % Epoxy/54 wt. % GORE-TEX®expanded PTFE fiber.

    ______________________________________                                                              A    B                                                  ______________________________________                                        Flexural Strength × 10.sup.3 psi                                                                12     20                                             Flexural Modulus × 10.sup.3 psi                                                                 250    930                                            Thermal Coefficient of Expansion ppm/°C.                                                       52     16                                             ______________________________________                                    

EXAMPLE V

A fabric was woven using conventional procedures from GORE-TEX® expandedPTFE fibers and fiberglass fibers. The fabric construction had 64expanded PTFE fibers per inch in the longitudinal direction and 60, 150I/O, fiberglass fibers per inch in the transverse direction. Ten, 5inch×6 inch sheets of the fabric were treated with TETRA-ETCH® andconverted into an epoxy B-stage pre-preg using the procedure describedin Example I.

A double-sided copper clad laminate was prepared from the pre-pregsheets using the procedure described in Example II, with the exceptionthat the pre-preg sheets were alternately stacked at right angles to oneanother so that fiberglass fibers were running in both the x and y axesof the finished, cured laminate.

The laminate had a core thickness of 0.075 inches and was composed of 36wt. % epoxy, 40 wt. % expanded PTFE fiber, and 24 wt. % fiberglass. Thedielectric constant at 1 MHz was 3.4 and the mechanical properties alsoimproved to the same degree as the fiberglass-containing sample ofExample IV, as demonstrated by the samples thermal coefficient ofexpansion of 14 ppm/° C.

EXAMPLE VI

A fabric was woven using conventional procedures from 400 denier drawn,solid PTFE fiber, available commercially from E. I. DuPont under productdesignation Teflon® TFE-Fluorocarbon Fiber. These fibers aremultifilament fibers and are not expanded. The fabric construction had60 fibers per inch in the longitudinal direction and 64 fibers per inchin the transverse direction.

Liquid epoxy resin would not wet the untreated PTFE fabric, but when thefabric was treated with TETRA-ETCH®, it was able to be processed by themethods described in Examples I and II. The DuPont PTFE fabric exhibitedthe same 20% shrinkage as the GORE-TEX® expanded PTFE fiber fabric, butlikewise could be stretched back to its near original size.

A double-sided copper clad laminate was fabricated using the methodsdescribed in Examples I and II. The laminate core was 0.088 inch thick,had a dielectric constant at 1 MHz of 2.6 and was composed of 32 wt. %epoxy resin and 68 wt. % DuPont fiber. The mechanical properties wereslightly lower than laminate fabricated from the GORE-TEX® expanded PTFEfiber.

Core Composition A=32 wt. % Epoxy/68 wt. % DuPont PTFE fiber.

Core Composition B=55 wt. % Epoxy/45 wt. % GORE-TEX® expanded PTFEfiber.

    ______________________________________                                                              A    B                                                  ______________________________________                                        Flexural Strength × 10.sup.3 psi                                                                7      12                                             Flexural Modulus × 10.sup.3 psi                                                                 250    250                                            Thermal Coefficient of Expansion ppm/°C.                                                       58     52                                             ______________________________________                                    

EXAMPLE VII

Four hundred denier fibers of GORE-TEX® expanded PTFE were treated withTETRA-ETCH® napthanate solution before weaving into a fabric as opposedto treating the fabric after it was woven as described in Example I. Thefiber was treated and washed in warm water while under longitudinaltension so as to prevent fiber shrinkage resulting in a more stablefabric construction. A fabric was woven from the fiber using a hand loomto minimize abrasion of the fibers. The fabric was then coated withliquid epoxy resin and converted into a pre-preg by the methodsdescribed in Example I. The pre-preg was well wetted by the epoxy resinand showed no evidence of voids or poor adhesion.

EXAMPLE VIII

A fabric was woven from 100 denier GORE-TEX® expanded PTFE fibers usingconventional procedures. The fabric had 80 fibers per inch in both thelongitudinal and transverse directions. The fabric was treated withTETRA-ETCH® alkali napthanate solution and stretched as described inExample I. The fabric at this point was approximately 0.005 inch thick.The fabric was passed between stainless steel calendering rolls set atapproximately a 0.002 inch gap. After two passes through the calenderingrolls, the fabric thickness was reduced to 0.0026 inches thick.

The fabric was converted into a pre-preg and double-sided copper cladlaminate using the methods described in Examples I and II. The laminatecontaining the sheets of calendered fabric had a core thickness of 0.005inch, which is a very desirable thickness for laminate used for multiplelayer printed circuit boards. The dielectric constant at 1 MHz was 2.8.

EXAMPLE IX

A fabric of 400 denier GORE-TEX® expanded PTFE fiber was woven andtreated with TETRA-ETCH® alkali napthanate as described in Example I.The fabric was bleached to nearly its original white color by immersingthe fabric for 5 minutes in Chlorox® chlorine bleach at 175° F.-200° F.The bleached fabric was then coverted into an epoxy pre-preg and unclad,cured laminate using the methods described in Example I.

The bleached fabric was wet by the liquid epoxy resin and the curedlaminate was free of voids. Adhesion between the bleached fibers andcured epoxy resin was excellent.

EXAMPLE X

A double sided copper clad laminate was made using the methods describedin Examples I and II except that two sheets of fabric woven from 100denier GORE-TEX® expanded PTFE fibers were utilized. The core thicknesswas 0.008 inch and the dielectric constant at 1 MHz was 2.6.

The laminate was tested for flexibility by folding it back on itselfwith a 0.0 inch radius. While the copper foil cracked at the bend, thedielectric core maintained its integrity and did not crack or separatewith up to 10 flexures. A double-sided copper clad commercial gradeepoxy/fiberglass laminate, with a 0.006 inch thick core, was also bentwith a 0.0 inch radius. The copper foil on the outside radius of thelaminate cracked, but the epoxy/fiberglass core also cracked and after 2flexures broke and separated into two parts.

EXAMPLE XI

Double-sided copper clad laminate and pre-preg were produced using themethods and conditions described in Example VIII. Three of the copperclad laminates were fabricated into double-sided innerlayer printedcircuits using standard procedures. The three innerlayer printedcircuits were interleaved with two sheets of pre-preg between each twoinnerlayer circuits for a total of four sheets of pre-preg in the stack.The stack was laminated under heat and pressure, imaged and plated usingconventional procedures for fabricating multilayer printed circuitboards.

The completed multilayer printed circuit board was free of voids andexhibited good intralayer adhesion.

EXAMPLE XII

A fabric was woven using the materials and procedures described inExample I. The fabric was cut into 8 inch×8 inch squares and placed overa 6 inch×6 inch pin frame (tenter frame). The pin frame was a square,window frame type structure which had 1/2 inch long pins placed on 1inch centers around the perimeter of the frame. The fabric was pressedonto the pins on one edge of the frame and stretched under moderatetension to the pins on the other side of the frame. The other two sidesof the fabric square were likewise stretched and placed on the pins onthe other remaining two sides of the pin frame. The fabric was thusrestrained under moderate tension on all four sides.

The pin frame holding the fabric was immersed in an alkali napthanatesolution for 30 seconds, washed in warm water and rinsed in acetone. Thefabric developed the characteristically dark brown color, indicatingeffective treatment, but it did not shrink appreciably after beingremoved from the pin frame. The fabric was coated with epoxy resin andprocessed as described in Example I to a pre-preg and then into alaminated composite. The epoxy wetted the fabric acceptably and thefully cured laminated composite performed equivalently to the laminatedcomposite of Example I.

While the invention has been disclosed herein in connection with certainembodiments and detailed descriptions, it will be clear to one skilledin the art that modifications or variations of such details can be madewithout deviating from the gist of this invention, and suchmodifications or variations are considered to be within the scope of theclaims hereinbelow.

What is claimed is:
 1. A process for making a dielectric materialcomprising:(a) treating fluorocarbon fibers to render them wettable byan uncured thermosetting resin, the fibers being held under longitudinaltension during such treatment, thereby preventing longitudinal shrinkageof said fibers during treatment to produce treated fluorocarbon fibers,(b) weaving a fabric containing at least a portion of said treatedfluorocarbon fibers, (c) impregnating said fabric with an uncuredthermosetting resin, and (d) heating and drying saidresin-impregnated-fabric to cure said resin to at least the semi-cured,B-stage state.
 2. A process for making a dielectric materialcomprising:(a) treating a fabric containing at least a portion offluorocarbon fibers to render said fluorocarbon fibers wettable by anuncured thermosetting resin, (b) impregnating said fabric with anuncured thermosetting resin, and (c) heating and drying saidresin-impregnated-fabric to cure said resin to at least the semi-cured,B-stage state.
 3. The process of claims 1 or 2 including the step ofcalendering said fabric to reduce its thickness.
 4. The process ofclaims 1 or 2 in which said treating includes applying an alkalinaphthanate solution to said fluorocarbon fibers or fabric.
 5. Theprocess of claim 4 including napping the fluorocarbon fibers to improvewettability and adhesion between said fibers and resin.
 6. The processof claims 1 or 2 wherein said fibers or fabric are bleached subsequentto treating them to make them wettable.
 7. The process of claim 6wherein an oxidizing agent is used to bleach the fibers or fabric. 8.The process of claim 7 wherein the fibers or fabric are bleached byheating to a temperature in the range of about 500° F. to about 675° F.9. The process of claim 2 wherein said fabric is held in longitudinaland transverse tension during said treatment, thereby preventingshrinkage of said fabric in the longitudinal and transverse directionsduring treatment.
 10. The process of claim 2 wherein said fabric isunrestrained during said treatment and is allowed to shrink, followingwhich the fabric is stretched to approximately its original dimensionsprior to said treatment.
 11. The process of claim 1 including laminatingan electrically conductive foil to at least one side of saidresin-impregnated-fabric.
 12. The process of claim 2 includinglaminating an electrically conductive foil to at least one side of saidresin-impregnated-fabric.
 13. The process of claim 1 or 2 includinglaminating one or more sheets of said resin-impregnated-fabric in whichthe thermosetting resin is cured to the fully-cured, C-stage state, andone or more sheets of said resin-impregnated-fabric in which thethermosetting resin is cured to the semi-cured, B-stage state, wherein,wherever a layer of material in said C-stage state contacts anotherlayer, said other layer is of material in said B-stage state.
 14. Theprocess of claim 1 or 2 including laminating one or more sheets of saidresin-impregnated-fabric in which the thermosetting resin is cured tothe fully-cured, C-stage state, and having an electrically conductivefoil bonded to at least one side of at least one of said sheets, and oneor more sheets of said resin-impregnated-fabric in which thethermosetting resin is cured to the semi-cured, B-stage state, wherein,wherever a layer of material in said C-stage state, and wherein,wherever an electrically conductive foil contacts another layer, saidother layer is of material in said B-stage state.
 15. The process ofclaim 1 or 2 including curing the thermosetting resin in the dielectricmaterial to the fully-cured, C-stage state.
 16. The process of claim 1or 2 including laminating at least one additional layer of a compositestructure other than said resin-impregnated-fabric to saidresin-impregnated-fabric.
 17. The process of claim 16 wherein said atleast one additional layer is a layer selected from the group consistingof epoxy/fiberglass, fluorocarbon/fiberglass, polyimide/fiberglass,epoxy/polyaramide fiber, epoxy/quartz fiber and polyimide/quartz fibercomposites.
 18. The process of claim 1 or 2 wherein said thermosettingresin is selected from one group consisting of epoxy, polyimide,polyamide, polyester, phenolic and acrylic thermosetting resin.